This project will explore the role of the Supplementary Motor Area (SMA) and dorsal Premotor cortex (dPMC) in the postural preparation and execution of voluntary stepping with and without visual targets in healthy and Parkinson's disease (PD) human subjects. The excitability of these cortical regions will be 'temporarily depressed by repetitive transcranial magnetic stimulation (rTMS). Measures of step timing and placement, prepatory shifts in foot pressure, and electromyographical records of muscle activity will provide data regarding the effect of PD, visual condition, levodopa, and rTMS on step preparation and execution. Hypotheses: (1.) The SMA is important for movement preparation and postural coordination prior to a voluntary step, and the symptom of bradykinesia (slow step initiation and impaired preparatory shifts in foot pressure) in PD is associated with impaired activity in the SMA. (2.) The dPMC is important for integrating visual input to modify voluntary steps (step placement and trajectory), and PD subjects exhibit an increased dependence on vision and the activity of the dPMC to execute voluntary steps.